Personal Alarm/Torch Project.. help?

Hi, brand new here.. really hope i can get some assistance.

Aight... basically, I messed about for the year and didnt learn as much as I should have. Have had to do a project designing a circuit for a personal alarm. I found the personal alarm circuit design online and added 2 led's to the circuit for the torch.

I then added 2 components (a resistor to limit the voltage to the led's & a diode to protect the oscillator) but i dont know what type they have to be.

If any1 has any knowledge on this and can help me it would be appreciated as I need to do this to pass my course.

(Excuse the old resistor symbols but thats what we were taught).

Complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker

big k said:

Hi, brand new here.. really hope i can get some assistance.

Aight... basically, I messed about for the year and didnt learn as much as I should have. Have had to do a project designing a circuit for a personal alarm. I found the personal alarm circuit design online and added 2 led's to the circuit for the torch.

I then added 2 components (a resistor to limit the voltage to the led's & a diode to protect the oscillator) but i dont know what type they have to be.

Click to expand...

For a start you shouldn't have only one resistor feeding two LED's - LED's shouldn't be put in parallel!. As it's only a three volt supply you can't put them in series, so you should use a seperate resistor to feed each LED.

You simply work out the resistor values with ohms law, you need to decide what current you want, then calculate the voltage across the resistor by subtracting the forward voltage drop across the LED (at that current) from the supply voltage.

If any1 has any knowledge on this and can help me it would be appreciated as I need to do this to pass my course.

(Excuse the old resistor symbols but thats what we were taught).

Complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker

Click to expand...

Asuming you want to add a diode?, use a 1A rectifier, one of the 1N4001 series would be fine - but I don't see as the circuit needs it anyway?.

Thanx alot for your help!

I don't think the circuit will work with R1 and R2 mixed up so I corrected it.

Thanks for spotting the resistors were labeled wrong. I hadn't noticed and was just about to print off ready to hand in!

So the diode really isn't needed? I wasn't too sure myself but I checked it with a teacher and he said it was :S

The small inductance of the speaker might cause a small negative "flyback" voltage spike across it when Q2 turns off. The diode in series with it won't make any difference since the diode will remain forward biased and pass the spike to the transistor anyway. If a protection diode is added to the circuit it must be across the inductance, not in series with it.